In a process automation system, a control loop typically consists of a process, a measurement, a controller and an actual control element, such as a valve, and a related device, such as a valve controller (positioner) and an actuator. An optimal process control depends on how appropriately all these components function.
In the processing industry, such as the pulp and paper industry, and petroleum refining, petrochemical and chemical industries, various control valves mounted in the plant pipe system control material flows in the process. A material flow may contain any fluid material, such as flowing subs stances, liquors, fluids, gases or vapours. At its simplest, the control valve may be a manually-controlled mechanical valve. Usually, the valve is provided with a valve controller and an actuator. The valve controller and actuator adjust the position of a control valve according to the control input (e.g. pneumatic or electric control input) received from the process control system.
FIG. 1 exemplifies a functional block diagram of a control valve. A valve controller (i.e. positioner) 10 controls the travel/position (h) of the valve by means of the torque generated by an actuator 11. Position information (h) is provided as feedback from the actuator 11 or valve 12 to an adder arranged at the input of the valve controller. The function of the valve controller is mainly based on an error (e) between an input signal u (control signal from the process) and a feedback position (h). The valve controller 10 minimizes this error by a control algorithm, such as a state or PID algorithm. This control algorithm is tailored for each valve and, if necessary, it can be tuned during the installation or operation. The tuning may include changing gain parameters. It is also feasible to use one or more additional feedbacks in the valve controller 10, such as speed or pressure feedback from the actuators cylinder, to obtain a more balanced and accurate control function of the valve position.
FIG. 2 illustrates a typical model of a process control loop which controls one control valve 22 and therethrough one material flow in a process. The control valve 22 may be similar to the one shown in FIG. 1, for instance. The process control loop includes a process controller 21, which is provided with a control algorithm, which produces a control signal u for controlling the control valve 22 according to a set point r (which is obtained from a control room computer, for example) and a feedback process variable y. The control algorithm may be any algorithm that is used in control systems, such as PID, PI or P control. The control signal u fed to the control valve 22 controls the valve position and travel and thus the material flow in the process. A desired process variable y is measured by a measurement transmitter 24 and it is compared (block 20) to the set point r of the same process variable to produce an error signal e2, which is fed into the process controller 21. The process controller 21 changes the control signal to minimize the control error 2e. The process control error typically results from changes in a positioning error and process interference.
The valve and its auxiliary devices often constitute the weak link in the control loop since they are the only moving parts. This movement causes problems, which decrease the capacity of the control loop. To avoid a backlash resulting from mechanical adjustments, the valve, actuator and valve controller/positioner have to be provided with mechanical tolerances that are sufficiently tight. As a result of the backlash, the valve movement does not follow the control signal accurately but deviates from it. The influence of the backlash becomes apparent in particular when the valve control direction and thus the valve's direction of movement are reversed. In that case, the control signal value keeps changing for a while until the measured output signal starts to change noticeably. This is also known as the dead band of control. In addition to the backlash in an actuator or positioner, this phenomenon may result from sticking of the valve or other mechanical factors, such as initial friction. The backlash between mechanical parts naturally increases as the parts wear.
The backlash and other error factors cause hysteresis between the control of the process device, such as a valve and/or its auxiliary devices, and the measured response. This is illustrated in FIG. 3. Straight line 31 illustrates an ideal relation, i.e. characteristic curve, between the control u and the measurement (output) y, such as valve position. The real dependency between the measurement and the control is illustrated by characteristic curve 32. As appears from FIG. 3, due to the backlash and any other factors, the upward control (increasing u) has a characteristic curve 32A different from that of the downward control (decreasing u), which has characteristic curve 32B. The difference between the curves represents hysteresis in the control of the process device.
In some cases, the controllers are provided with automatic backlash compensation, which attempts to take the mechanical non-ideality of the device into account always when the control direction is reversed. This approach is described in U.S. Pat. No. 5,742,144, for example. Approach of this kind is good in theory but limited in practice since the backlash and hysteresis vary due to different factors.
The information on hysteresis and backlash is, however, important to the tuning of the control circuit. It also gives useful information on the condition of the process device, such as a valve and/or its actuator or valve positioner. If hysteresis or backlash increases significantly, service measures can be taken to fix the matter.
A typical way of detecting the hysteresis or backlash of an actuator is to switch the controller to a manual controlling mode and perform a sequence of step tests. In that case, the actuator is driven to the same position from different directions, in which case any differences between the control and the response due to backlash or hysteresis are found out by means of measurements. Another typical way is to drive the actuator back and forth over the whole control area and to estimate backlash and hysteresis from the measurement results. In the case of a valve actuator, for example, the valve is driven from the closed position to the open position and back to the closed position. A problem associated with these solutions is, however, that they are separate tests that need to be carried out when the process is interrupted or the process device to be examined is bypassed or detached from the process. Similar tests that are performed on the valve positioner are described in IEC (International Electrotechnical Commission) standard 61514, Industrial process control systems: Methods of evaluating the performance of valve positioners with pneumatic outputs, first edition, 2000-04.
WO 01/11436 discloses a method and an apparatus which statistically determine estimates for one or more process control loop parameters for the device or the control loop that is active in the process control environment. Such parameters include friction, dead band, dead time, vibration or backlash. In the method, one or more signals are always measured in the process control loop when the process control loop is connected to the on-line process control environment. The measured signal is stored as signal data, after which a number of statistical analyses are carried out on the stored data to determine the desired parameter estimate. An advantage of this solution is that the process device does not need to be removed from process or the control loop bypassed for the test.
In practice, the on-line determination of hysteresis or backlash is sensitive to process interference as well as inaccurate. Furthermore, it usually requires statistical calculation methods, matrix calculation, mathematical functions, etc.